Characteristics of rare earth elements of geothermal water from Fengshun-Tangkeng geothermal field and its affecting factors

Abstract

Geothermal resources and multi-phases granites are widely developed in igneous rock regions of South China and they have close relationships. Studies on differentiation properties and enrichment mechanism of rare earth elements (REE) of granite rock have been done a lot while that for geothermal water is seldom. Taking Fengshun-Tangkeng geothermal field as the study area, this paper discusses properties and species of REE of geothermal water and its affecting factors. Results show that geothermal water has lower REE content of 0.020-0.204 ?g/L comparing with shallow groundwater and Rongjiang River water, average concentrations of which are 0.275 ?g/L and 0.183 ?g/L, respectively and it is enriched by MREE. It exhibits REE distribution pattern of positive Eu anomaly and negative Ce anomaly with reference to PAAS, which are caused by silicate minerals dissolution of parent rock under alkaline conditions. REE content show positive correlations with pH and also its species are controlled by pH. Calculations by PHREEQC indicate that inorganic species of REE in alkaline geothermal water are dominated by bicarbonate complex of Ln?CO3?2- and monocarbonate complex of LnCO3+ with minor content of LnF2+ and Ln3+ (Ln refers to REE). Total proportions of Ln?CO3?2- and LnCO3+ can exceed 95%. In contrast, REE species in neutral Rongjiang River water and slightly acid shallow groundwater are mainly composed of LnCO3+ and Ln3+ with certain content of LnSO4+, respectively, average proportions of LnCO3+ can be up to 86.8%. Besides, with increase of atomic number, concentrations of Ln3+ complex decrease until becoming 0.